In the fabrication of packaging for integrated circuits, the technology has moved in the direction of dielectrics having low co-efficients of thermal expansion, such as the glass-ceramic compositions taught by Herron, et al in U.S. Pat. No. 4,234,367 and Kumar, et al in U.S. Pat. No. 4,301,324 (which are assigned to the present assignee and hereby incorporated by reference). The co-efficients of thermal expansion for the crystallizable glass-ceramic compositions disclosed therein, B-spodumene (Li.sub.2 O.multidot.Al.sub.2 O.sub.3 .multidot.4SiO.sub.2) and cordierite (2 MgO.multidot.2Al.sub.2 O.sub.3 .multidot.5SiO.sub.2) more closely match that of silicon, of which the integrated circuits are fabricated. As discussed in the above-referenced patents, use of the glasses having lower firing temperatures necessitates the use of metallurgy, which will sinter at or about the crystallization temperature of the glass. Copper-based metallurgy is preferred given its low sintering temperature, good conductivity and reasonable cost and availability.
To fabricate a package of a crystallized glass-ceramic dielectric having associated copper-based metallurgy, one employs the early alumina "greensheet" technology in which a slurry of the glass-ceramic precursors and polymeric binders is cast into sheets, dried, embossed with a pattern for the metallization and metallized with a paste comprising the conductive metal (or a precursor thereof) and at least one organic vehicle. When a plurality of sheets have been appropriately patterned and metallized, the sheets are stacked and laminated into a green (unfired) package. Firing is conducted following a prescribed firing profile to assure the heating and atmospheric conditions sufficient to effect burn-off of the relevant organics, maximum densification and crystallization of the glass-ceramic with attendant sintering of the metallurgy.
The drawbacks associated with the choice of a copper-based metallurgy include the constrained firing profile, requiring an atmosphere which is non-oxidizing to the copper, whereas an oxygen-rich atmosphere would be most advantageous for effecting removal of the organics. In addition, copper does not adhere well to the dielectric materials, leaving voids which compromise hermeticity and connectivity in the finished product. Moreover, the rate of shrinkage of the copper differs significantly from that of the dielectric, again affecting both the electrical properties and the hermeticity of the package. Finally, and most detrimentally, the pure copper powders can begin to sinter and shrink at temperatures as low as 400.degree. C., far below the temperature for the onset of coalescense of the glass-ceramics. Premature sintering can lead to severe distortion of the package and the device-threatening effects of trapping of gases from the volatizing organic carriers.
Those skilled in the art have attempted to address each of the above-identified concerns individually. Prabhu, et al, in U.S. Pat. No. 4,816,615, teach the addition of an inert glass frit to the conductor paste to delay sintering for a length of time sufficient to allow the organic vehicle in the paste to burn off, thereby minimizing voids and the other undesirable effects of trapped contaminants. An obvious drawback of glass frit additions is that the conductivity of the metallurgy is compromised.
The adhesion concern, noted above, has also been addressed by the addition of glass or ceramic compositions. Rather than as inert fillers, however, the adhesion promoting additions are chosen to bond to the metal of choice. The Pryor, et al. U.S. Pat. No. 3,676,292 teaches the use of Al.sub.2 O.sub.3 coatings on copper particles to promote adhesion of the copper metallurgy to the glass or ceramic workpiece. The teachings of Siuta, in U.S. Pat. No. 4,594,181, also employ metal oxide coatings on copper metallurgy. The Siuta additives are introduced to delay sintering and also to reduce the shrinkage of the metallurgy to .about.15% to more closely match the overall shrinkage of the glass-ceramic package. The Siuta method is to coat copper metal powder with a metal oxide by first coating the copper particles with an organometallic compound followed by heating in a reducing atmosphere to decompose the organometallic, thereby leaving a continuous metal-oxide coating on the copper. The preliminary processing has the obvious drawback of cost and processing time constraints. Moreover, there may be contaminants, present due to the preliminary processing, which have the potential of affecting both subsequent processing and end-product effectiveness.
The methods noted above have inherent drawbacks. Moreover, no one solution has been effective in addressing all of the challenges of the glass-ceramic/copper package processing, namely, shrinkage, premature sintering and adhesion.
It is, therefore, an objective of the present invention to provide additions to a metallizing paste which will not only delay sintering of the paste and match shrinkage of the paste to that of the glass ceramic, but also promote adhesion of the copper-based metallurgy to the glass-ceramic.
It is a further objective of the invention to provide a simplified method of delaying sintering and retarding shrinkage in copper-based metallurgy.
It is yet a further objective of the present invention to provide a metallizing paste which, upon firing, exhibits good adhesion to the associated glass-ceramic and which does not require glass additions to the metallizing paste.
It is still another objective of the subject invention to provide a sintering retardant to a metallizing paste which does not require undue processing prior to mixing the paste.
Yet another objective of the present invention is to provide paste additions which will not significantly compromise the electrical properties of the conductors.